The present invention relates to the general technical field of devices for protecting electrical equipment or installations against electrical disturbances, in particular against transient overvoltages due in particular to a lightning strike. This invention relates more particularly to a protection device, such as a varistor-type surge arrester, associated with or intended to be associated with an electric cut-off device such as a circuit breaker.
It is known to ensure overvoltage protection of an electrical installation by means of devices including at least one overvoltage protection component, in particular one or more varistors and/or a spark gap. In the most frequent cases, a varistor is hooked up between one phase and the neutral conductor of the electrical installation while a spark gap is connected between the neutral conductor and ground.
In the event of failure of the or one of the protection components, these devices include a disconnection system serving to isolate the protection component or components of the electrical installation as a safety measure.
In particular, in the case of a varistor connected between one phase and a neutral conductor, it is conventional to provide a thermal protection, which is required by the international standards applicable to these devices. The thermal protection serves to disconnect the varistor of the electrical installation being protected in the event of overheating of the varistor, e.g., above 150° C. This overheating of the varistor is due to the increased leakage current therethrough—generally a few tens of milliamperes—due to the ageing of same. In this case, reference is made to the thermal runaway of the varistor.
Thermal protection often consists of one or more low-temperature welds elastically holding a restraining element in place, the melting of the weld or welds enabling the movement of this element with the effect of opening the circuit of the varistor. Thermal protection devices of this type are described in particular in EP-A-0 716 493, EP-A-0 987 803 and EP-A-0 905 839.
Sometimes, thermal protection is based on an electronic measurement of the current, as described, for example, in FR-A-2 873 510, which has the disadvantage of being very costly.
Protection can also be provided specifically against short-circuits and separate from the thermal protection. It serves to disconnect the varistor in the event of a complete short-circuiting thereof, e.g., subsequent to a significant lightning strike. This generally involves a thermomagnetic circuit breaker.
Whether it be for thermal protection or for protection against short-circuits, it is generally provided for the disconnection of the varistor to occur without causing the general cut-off members of the electrical installation to open, so as to ensure a continuity of service of the electrical installation.
The disadvantage is that the thermal protection and the protection against short-circuits are separate and each use a respective cut-off device. That of the thermal protection can have a low breaking capacity while that of the short-circuit protection must be capable of cutting off very high currents. However, the fact of using two cut-off devices has the disadvantage of both increasing the spatial requirements of the protection device and the cost thereof.
Thus, an overvoltage protection device was proposed in EP-A-1 607 995, comprising a protection module and a circuit breaker. The protection module comprises a varistor and a spark gap, which are connected to the electrical network being protected by the circuit breaker. In order to ensure disconnection of the varistor and the spark gap in the event of the failure of one of them, the protection module includes separating means in order to cause the circuit breaker to open. More precisely, these separating means consist of a thermal pin arranged on an area in thermal connection with the varistor and an electric fuse connected in series with the spark gap. The thermal pin is made of a metal alloy or of a material which is thermofusible at a low melting temperature. When the pin or the fuse melts or breaks, a mechanical actuating system ensures that the circuit breaker is triggered and, as a result, that the varistor and spark gap are disconnected from the electrical network. More particularly, the mechanical actuating system includes a lever, which is connected to the thermal pin, and another lever, which is connected to the fuse, these levers being pulled by a respective return spring. In the event that the pin or the fuse melts, the corresponding lever acts on a control centraliser under the influence of the return spring, the control centraliser actuating the circuit breaker triggering mechanism by means of a mechanical link.
However, this device has several disadvantages. A pin made of a thermofusible material is thus not very precise as concerns the temperature at which it melts or breaks and therefore causes the circuit breaker to be triggered. The metal alloy pin having a low melting temperature ensures a higher degree of precision, but, besides the higher cost thereof, has the disadvantage of being very difficult to produce and generally contains lead or cadmium-type polluting materials.
In addition, EP-A-1 447 831 describes a device for protecting against overvoltages due to lightning, by combining a lightning arrester block and a thermomagnetic circuit breaker, the lightning arrester block comprising a varistor. According to one embodiment, the lightning arrester block includes a thermal disconnector, which is thermally connected to the varistor 12. The thermal disconnector consists of a low-temperature weld cooperating with an elastic strip triggering the circuit breaker after the weld melts under the effect of the heat from a thermal runaway of the varistor. This embodiment has disadvantages similar to those previously mentioned with regard to the thermal pin made of a metal alloy.
According to another embodiment, this document teaches to thermally connect the bimetallic strip of the circuit breaker to the varistor. If the bimetallic strip of the circuit breaker is not sufficiently deflected when the currents appearing during the thermal runaway of the varistor pass therethrough, the thermal connection between said varistor 12 and the bimetallic strip produces sufficient deflection to cause the circuit breaker to be triggered, the latter of which disconnects the lightning arrester block from the electrical network. However, this embodiment also has disadvantages. In particular, it is not possible to use conventional commercial circuit-breakers because they are not intended to enable the bimetallic strip thereof to be thermally connected to an element outside the circuit breaker box. Such a device requires modification of the circuit breaker design in order to be capable of effectively conveying the heat released by the varistor to the bimetallic strip of the circuit breaker. Furthermore, the circuit breaker cannot be freely chosen, taking into account the fact that the bimetallic strip thereof must be designed to cause the circuit breaker to be triggered at a given critical temperature reached by the varistor.
An overvoltage protection device is also known from WO 2004/064213, which comprises a varistor and a means for breaking the electric current passing through the varistor. In one alternative, this breaking means includes a sliding rod holding an electrical contact element enabling the electrical circuit of the varistor to be opened or closed. During normal operation, the rod is pre-stressed in the closed position of the contact by means of stop-motion device in the form of a plate arranged at the end of a bimetallic strip. The bimetallic strip is mounted and positioned in the device so as to be sensitive to the heat released by the varistor. In the event of overheating, the bimetallic strip bends so as to disengage the stop-motion device in order to release the rod, which is pushed by a spring towards the open position of the contact. In another alternative, the varistor is powered via the bimetallic strip and a conductive element arranged at one end of the bimetallic strip. During normal operation, this conductive element is in electrical contact with a connector, thereby enabling the varistor to be powered. In the event of overheating, the bimetallic strip bends so as to distance the conductive element from the connector, the effect of which is to shut off the electrical power supply of the varistor. In addition, an insulating shield is placed between the conductive element and the connector in order to prevent reclosing of the circuit when the bimetallic strip returns to the initial position thereof after cooling.
In the two alternatives, these devices have the disadvantage of requiring a meticulous bimetallic strip design, as well as good mounting accuracy. As a matter of fact, dependent upon this is the contact force applied to the electrical contact of the breaking means during normal operation, which must be capable of conducting very high currents in the event of lightning-related overvoltages on the electrical network. Furthermore, the deformation tolerance of the bimetallic strips relative to temperature require an individual adjustment similar to that implemented to adjust the calibres of modular circuit breakers. Furthermore, the slow and gradual opening of the contacts does not enable a short-circuit current to be cut off.